A twin study will be carried out to investigate two long-standing questions in the study of language development: (1) How large a role does experience (as opposed to innate knowledge) play? and (2) Does language require two distinct kinds of mental computations: rules and associations? The twin method, a behavior genetic technique that uses identical and fraternal twins to estimate the relative contributions of heredity and environment to variation in a trait, can help to resolve these issues. While the twin method has been applied in many areas of psychology, it has not yet been used to its potential in language development. The first question is addressed by studying the development of the passive voice, a construction that is not mastered by children until quite late (age 4-7). The twin method is used to determine to what extent genetic variance and environmental variance (i.e., differences in language input) are each responsible for the delay. The second question is addressed by studying the development of regular and irregular past tenses, along with other, similar morphological contrasts, as example cases of rules versus associations. The twin method is used to specify the heritability of development of each of these types of constructions (rule-based ones versus association-based ones), and then the heritabilities are compared. A substantial difference would provide further support for the notion that these processes are learned in different ways and accomplished in different ways by adults.